As the need for higher data rates increases, communication systems are being designed to cover wider frequency bandwidths as well as a larger number of frequency bands. The introduction of 4G protocols such as Long Term Evolution (LTE) are a main driver in the increase in additional frequency bands being used for cellular communication systems. The complexity of the RF front-end topology of communication systems is increasing due to the need for backward compatibility with 2G and 3G protocols as 4G LTE capability is introduced. In addition, Advanced LTE as a protocol is configured to accommodate carrier aggregation, where multiple channels can be transmitted or received on simultaneously to increase instantaneous bandwidth. This aggregation of channels can cover up to five channels spread across multiple frequency bands. Carrier aggregation that utilizes multiple frequency bands points to a need for dynamic tuning of various components of the RF front-end including the filters to provide the flexibility needed to access various frequency pairings. All of these trends point toward a growing need for more flexibility in the RF front-end of mobile communication systems to address the combining of multiple frequency bands and modes.
Dynamic tuning of components that comprise the RF front-end of communication systems is picking up adoption in the commercial communications industry, and proper implementation of dynamic tuning methods can bring improvements to communication system performance as the number of frequency bands that can be accessed grows and the instantaneous bandwidths required increases.
The requirement to design a mobile device such as a cell phone that covers multiple frequency bands and multiple modes of operation forces the system designers to develop a front-end transceiver circuit that combines several power amplifiers (PA) on the transmit side and several low noise amplifiers (LNA) on the receive chain. Typically, each PA and LNA requires a filter to reduce spurious emissions and harmonics. Without the filters in the circuit, the PA will amplify unwanted frequency components which can fall within the frequency band of receive functions. On the receive side, the absence of filtering will cause the LNAs to amplify unwanted frequency components which can result in an increase in the noise floor in the receive chain. This increase in noise floor can result in reduced Signal to Noise Ratio (SNR) which in turn can result in an increase in Bit Error Rate (BER), with the end result being a decrease in data rate for data transmission.
The filters used in current commercial communication systems have a fixed frequency response. The start and stop frequencies which establish the instantaneous bandwidth as well as the center frequency are fixed. The conventional technique for implementing filters in a communication system is to determine the frequency bandwidth required from the filter along with the slope of the skirts (the roll-off in performance as a function of frequency) for a specific function and location within the circuit topology. A filter that meets the frequency response requirements is designed, manufactured, and implemented in the circuit. Very good filtering can be achieved using SAW (Surface Acoustic Wave), BAW (Bulk Acoustic Wave), and FBAR (Film, Bulk Acoustic Resonator) filter types in a fixed filter implementation. The main drawback is the inability to dynamically alter or tune a filter response once it is implemented in a circuit. A tunable filter would provide the capability of dynamically adjusting the bandwidth of a transmit or receive circuit to track changes in bandwidth for LTE waveforms. LTE provides for a range of bandwidths for a data stream based upon the amount of data needed to transmit or receive and priority of the data stream in the cellular network. LTE bandwidths can vary from 1.5 MHz to 20 MHz. With carrier aggregation being implemented in LTE-Advanced (LTE-A), there is now a potential of up to five channels being aggregated to increase instantaneous bandwidth to 100 MHz. A tunable filter would provide the capability of matching the bandwidth of the communication system front-end to the instantaneous bandwidth of the LTE waveform.